1. Field of the Invention
The present invention relates to a method and an apparatus for analyzing contaminative element concentrations, and more specifically to a method and an apparatus for measuring contaminative element concentrations on a semiconductor substrate, for instance with the use of an energy dispersive type total reflection X-ray fluorescence analysis.
2. Description of the Background Art
Conventionally, as a non-destructive contaminative element concentration analyzing apparatus, there is so far known a total reflection X-ray fluorescence analysis (see "Inspection, Analysis and Measurement Technology Required 16M/64M Integration or After", by Ayako SIMAZAKI, Kunihiro MIYAZAKI; NIKKEI MICRODEVICE, No. 86, pages 148, 154, 156 and 158, Aug., 1992). Furthermore, as a contaminative element concentration analyzing apparatus based upon total reflection X-ray fluorescence analysis, an energy or wavelength dispersive type apparatus is known. Since the contaminative element concentrations can be analyzed nondestructively with the use of the contaminative element concentration analyzer based upon the total reflection X-ray fluorescence analysis, it has become possible to manage the contamination of silicon wafer during the semiconductor manufacturing process, and thereby the contamination of wafer can be reduced effectively.
FIG. 5 is a conceptual block diagram showing an example of the contaminative element concentration analyzer using the energy dispersive type total reflection X-ray fluorescence analysis.
In FIG. 5, a sample supporting base 42 is mounted within a vacuum chamber 41, and a sample (e.g., silicon wafer) 43 is mounted on this sample base 42. An X-ray generated by a rotating pair-cathode type X-ray source 44 is converted to a monochromatic ray through a monochrometer 45, being passed through a slit 49, and then allowed to be incident upon the sample 43 at a small total-reflection angle. On the basis of this incident X-ray, a fluorescent X-ray can be generated from the surface of the sample 43. The generated fluorescent X-ray is detected by a detector (e.g., semiconductor detector ), and converted into electric signals corresponding thereto. The fluorescent X-ray signals detected as described above are processed by pulse processor 47 to obtain an observed waveform as shown in FIG. 6. In FIG. 6, the abscissa designates the energy of the detected fluorescent X-ray and the ordinate designates the signal intensity (relative intensity according to the number of photons incident upon the detector 46) of the detected fluorescent X-ray. FIG. 6 indicates that the observed waveform (graph) has peeks each of which has a value for each element (silicon and other contaminative elements) contained in the silicon wafer 43. In addition, the integral intensity (which corresponds to an area of a peak waveform) of each peak is proportional to the concentration of the element.
On the other hand, an arithmetic processing circuit 48 stores information indicative of the relationship between the integral intensity of the fluorescent X-ray and the concentration for each contaminative element, which referred to as "analytical curve". Therefore, the arithmetic processing circuit 48 first separates the peaks of the contaminative elements from the observed waveform (see FIG. 6) inputted by the pulse processor 47 for concentration detection, and then calculates the respective integral intensities of the separated peaks, and obtains the contaminative element concentrations on the basis of the integral intensities and the analytical curves.
A co-pending U.S. patent application 08/116,750 which is incorporated herein discloses a method of peak separation.
Here, the analytical curves have been so far prepared by actually measuring the contaminative element concentrations and the fluorescent X-ray intensity distribution (see FIG. 3). In more detail, the concentrations of predetermined contaminative elements are measured, and further the fluorescent X-ray distribution (see FIG. 3) of the same sample is detected. Further, each peak integral intensity is calculated, and the relationship between each contaminative element concentration and each integral intensity is plotted so as to form a graph (the ordinate: concentrations; abscissa: integral intensities) to prepare the analytical curve. Here, the concentrations of the contaminative elements can be measured in accordance with a destructive analysis, for instance using WSA (Wafer Surface Analysis) and AAS (Atomic Absorption Spectrometry) (see AAS: by A. Simazaki, ECS Proceedings, Defects in silicon II, Ed. M. Bullis et el., pp. 47, 1991).
In the conventional contaminative element concentration analyzer as shown in FIG. 5, however, since it has been necessary to form the analytical curves for respective contaminative elements, there exists such a problem in that it takes much labor to from the various analytical curves of various elements.
Furthermore, in the case of the atomic absorption spectrometry, since the element concentration must be measured only for each element, when the analytical curves of a number of different contaminative elements are required to form, it has been necessary to use the recovery solution obtained by the wafer surface analysis, after having been divided. Therefore, in the case of low concentration (less than 10.sup.9 atoms/cm.sup.2), since there exists a possibility that the measurement results disperse, the number of plots must be increased to obtain an accurate analytical curve, so that it takes much labor to prepare the analytical curve from this point of view.
Furthermore, since the intensity of the fluorescent X-ray detected by the detector 46 is of relative value, the relationship between the integral value of the fluorescent X-ray intensity and the element concentration is susceptible to the optical system. That is, when the optical system changes, the relationship between the tow also changes, with the result that the conventional contaminative element concentration analyzer involves such drawbacks that the analytical curves must be prepared again whenever the optical system is modified due to repair, for instance.